The aims of this study were: a) to investigate differences in muscle thickness (MT) measured by ultrasound and muscle quality (MQ) at three sites in the elbow flexors and knee extensors, and b) to correlate MT with isometric force production of the elbow flexors and knee extensors. Sixty-nine young male subjects participated in this study. The isometric force production values of the elbow flexors and knee extensors were obtained at 90° and 60°, respectively. MT was assessed at three sites in the elbow flexors (proximal - BB1, middle - BB2, and distal - BB3) and knee extensors (proximal - RI1, middle - RI2, and distal - RI3). Differences between the three measurements of MT and MQ were evaluated by one-way repeated measures ANOVA. Pearson's correlation test was used to assess the correlation between MT and isometric force. The level of significance was set at p<0.05. The results showed significant differences (p<0.001) between the three MT and MQ sites in the elbow flexors (BB1, BB2, BB3) and in the knee extensors (RI1, RI2, RI3). The correlation between BB1, BB2 and BB3 and force production was r=0.57, r=0.52 and r=0.59, respectively (p<0.05). In the knee extensors, the correlation between RI1, RI2 and RI3 and force production was r=0.47, r=0.51 and r=0.53, respectively (p<0.01). There were significant differences in MQ between elbow flexors (p<0.001) and knee extensors (p<0.001The present results showed that the site of measurement has a significant influence on MT and MQ. Furthermore, MT was positively correlated with isometric force production in the elbow flexors and knee extensors.